The invention pertains generally to a pressure measuring system and is more particularly directed to a high resolution pressure measuring system including a dual configuration of quartz capacitive transducers.
In many areas of technology the necessity of measuring a pressure for a control system is becoming prevalent. Particularly, in the fuel control of combustion engines one of the most important parameters to be measured is the mass air flow ingested by the engine. To accurately describe the mass air flow, the change in density of the air corresponding to pressure changes in the air flow must be calculated. Modern pressure measuring systems are utilized to measure these changes.
Fuel control systems that advantageously include these pressure measuring systems are those conventionally found on gas turbine engines for aircraft or other uses. Normally, the ambient air pressure, compressor inlet pressure, and compressor output pressure are measured. The measured pressures are input to highly complex fuel and position schedules of an electronic controller for calculating the amount of fuel supplied to the engine and for the control of the variable geometry of the compressor. It is understood that the precision with which the controlled parameters are regulated by the electronic controller depends to a large extent on the accuracy of the pressure measurement.
Furthermore, for a more facile interfacing with an electronic fuel controller, a digital output is desirable. More often today fuel controllers are digital electronic processor oriented and a direct digital output from the pressure measuring system would allow the input of the information to a digital processor without further conversion or waste of program time. A direct digital output additionally eliminates the need for a separate time shared or dedicated analog to digital converter.
To precisely control gas turbine engines accuracies of 1% of point over a pressure range of 100-to-1 are desired. This requirement contemplates that for any pressure reading over the range the accuracy of the measurement will be maintained at a constant 1%. However, this operation produces a distinct resolution and measurement problem in the digital context. Because the smallest increment of a digital reading must correspond to 1% of the lowest pressure in the range, it will thus correspond to only 0.01% of the highest pressure. This amount of resolution at the highest pressure is equivalent to .+-.1/2 parts out of 10,000 and thus would require digital components with at least a 15-bit capacity. Therefore, it would be highly desirable to provide a direct digital pressure measurement system with a 1% of point accuracy over a 100-to-1 range while using digital components with less than a 15-bit capacity.
A direct digital pressure measuring system that has been previously developed for providing 1% of point accuracy over a 100-to-1 pressure range is a closed loop feedback system comprising a pressure sensitive capacitor, a stable reference capacitor, and a stable feedback capacitor. The pressure sensitive capacitor and reference capacitor are excited by sinusoidal signals 180.degree. out of phase with each other. The outputs of both capacitors are connected to a summing junction thereby producing a differential signal which is proportional to the pressure. The summing junction output is used to excite the feedback capacitor connected in a closed loop manner back to the input of the summing junction. The output from the feedback capacitor is utilized to null the differential signal and is thus a measure of the pressure. Additionally, the feedback loop includes a counter means for direct digital measurement of the number of discrete increments of feedback signal necessary to null the output of the summing junction. The digital readout of the counter is directly proportional to the desired pressure measurement.
This pressure measurement system, while providing the desired accuracy, is expensive because of the necessity of a 15-bit counter and digital-to-analog converter to measure the increments of feedback signal necessary to null the loop. Further, the slew rate of this system is too fast at the lower pressure measurements and too slow at the higher pressure measurements. Therefore, it would be desirable to reduce the number of bits necessary for the accuracy requirement while retaining the advantages of the direct digital readout of pressure.